Goods-elevators for vehicles include a load carrier or platform, the movements of which are controlled by means of a system of piston-cylinder mechanisms, usually comprising on one hand a first pair of mechanisms with the purpose of controlling the raising and lowering movements of the plate, on the other a second pair of mechanisms with the purpose of providing tilting or pivoting movements of the platform into different tilted or inclined positions relative to the horizontal direction. Thus, these latter mechanisms have to provide for the tilting of the plate within an upper range from a raised vertical position coming into question during the travel of the vehicle to a position approximately horizontal or inclined upwardly from the horizontal direction, and from there further into a lower movement range limited by said position inclined upwardly from the horizontal direction and a position inclined, as desired, downwardly from the horizontal direction.
Piston-cylinder mechanisms for this purpose have usually operated with a constant speed, giving at all times substantially one and the same speed to the pivoting or tilting movement of the plate. Since this speed, for the reason of manipulation techniques etc., has to be relatively low in the lower movement range wherein the plate pivots between its position inclined upwardly from the horizontal direction and its downwardly inclined position, the fact that the piston-cylinder mechanisms operate with one speed solely involves that a low speed was necessarily obtained also in the upper movement range, whereby the total time required to raise and lower the plate respectively was comparably great. To the operator operating the goods elevator, who frequently works under narrow time limits, this unnecessarily long waiting time has been extremely irritating, and this circumstance has contributed greatly to delay and deteriorate the whole manipulation process in which the operation of the goods elevator is comprised as a stage.
To avoid as far as possible the above disadvantages the French Pat. No. 2,173,861 proposes to design the piston-cylinder of mechanism with a narrow piston rod telescopically movable in relation to the wider piston of the mechanism whereby the piston rod will be movable with two different speeds, in fact on one hand a comparably low speed when the piston as well as the piston rod move, on the other a comparably high speed when the piston rod alone moves. While this piston-cylinder mechanism per se permits a variation of the speed at which the elevator platform is pivoted between different swivelling positions, at least one serious disadvantage is involved thereto. Thus the piston rod is movable solely through an interlude between, on one side, the pressure liquid of the mechanism, and on the other the weight of the platform. More particularly, the raising of the platform is performed by introduction of pressure liquid into the operation chamber of the mechanism, while the lowering of the platform is effected by the proper weight of the platform, which is relied upon to return the piston rod into its bottom position. This involves in practice that the platform will at times lock itself in the raised position, since the platform has in some situations no possibility of influencing the piston rod of the mechanism through its own weight.